1. Field of the Invention
The present invention relates to a pneumatic tire, and in particular, to a pneumatic tire having a block pattern which can provide good off-road performance, on-snow performance, and on-road traveling performance.
2. Description of the Related Art
As shown in FIG. 5, in order to improve off-road traveling performance, tires for passenger and four-wheel drive vehicles generally employ a block pattern structure in which a tread 100 has circumferential grooves 102 which extend in zigzags in the circumferential direction of the tire in order to increase the number of the longitudinal and lateral edge factor, and has transverse grooves 104 which extend in the tire axial direction from the vertices or the sides of these zigzags.
To describe this tread pattern in more detail, the tread 100 is provided with a pair of circumferential grooves 106 which are formed parallel to a tire equatorial plane CL between the pair of circumferential grooves 102, and which are formed linearly and have constant widths.
A large number of transverse grooves 108, which are inclined with respect to the circumferential direction of the tire, are formed, substantially in the circumferential direction of the tire, at the region between the pair of circumferential grooves 106. Further, a plurality of center blocks 110 are formed thereby.
An auxiliary groove 112, which is narrow and is inclined in the direction opposite to the transverse grooves 108, is formed in the center block 110 such that two sub-blocks are formed. Further, a plurality of sipes 113 are formed in the center block 110.
A plurality of second blocks 114 are formed in the region between the circumferential groove 102 and the circumferential groove 106. An auxiliary groove 116, which is narrow and is inclined in the same direction as the transverse grooves 104, is formed in the second block 114, so as to form two sub-blocks. A plurality of sipes 118 are formed in the second block 114.
A large number of shoulder blocks 120 are formed at the outer sides of the circumferential grooves 102 in the widthwise direction of the tire. An auxiliary groove 122, which is inclined in the same direction as the transverse grooves 104, and a plurality of sipes 124 are formed in the shoulder block 120.
The transverse grooves 108 and the transverse grooves 104 at the left and right sides are inclined upwardly toward the right. Further, the center blocks 110, the second blocks 114, and the shoulder blocks 120 are disposed such that adjacent blocks have a phase difference in the circumferential direction of the tire therebetween.
Note that W in the drawings denotes the ground contact width.
In this type of pneumatic tire, in order to improve the on-road traveling performance, the rigidity is increased and the performances are maintained by making the blocks larger and by ensuring, with the negative ratio being 45% or less, the sufficient ground contact surface.
When the tread surface of the tread 100 is divided into a center tread region, which is located between the (two) circumferential grooves 102, and side tread regions, which are positioned at the outer sides of the center tread region in the widthwise direction, the pitch number ratio of the blocks of the center tread region and the both side tread regions is set to be 1:1.
In a conventional pneumatic tire such as that shown in FIG. 5, usually, in order to ensure the off-road and on-snow performances, a large number of longitudinal and lateral edge factor (sipes and the like) are formed in the tread.
However, an increase in the number of edge factor in the block leads to a decrease in block rigidity and relates to a deterioration in on-road performances.
Thus, in order to prevent the rigidity from decreasing, there are many cases in which the block is set to be large. However, when the blocks at the side tread regions of the tread surface are large, a problem arises in that pattern noise becomes markedly worse.
In recent years, as vehicles have become more high-power, the need has arisen for tires to exhibit even higher levels of on-road and off-road performances.
Thus, a pneumatic tire having the tread pattern shown in FIG. 6 was conceived as an improvement on the traveling properties of the pneumatic tire having the tread pattern shown in FIG. 5.
In the tread pattern shown in FIG. 6, reference numeral 12 is a tread; 14L, 14R are circumferential grooves; 16L, 16R, 24L, 24R, and 28 are transverse grooves; 18L, 18R are shoulder blocks; 34L, 34R, 36L, 36R, 38L, 38R, 41 and 42 are auxiliary grooves; 30L, 30R are second blocks; 32 is a center block; 22L, 22R, 40L, 40R and 44 are sipes; and W is the ground contact width.
The feature of the pattern shown in FIG. 6 is that the number of transverse groove components is greater than in the pattern shown in FIG. 5. Note that the center blocks 32 and the second blocks 30 have absolutely no portions which overlap in the tire axial direction when viewed in the circumferential direction of the tire.
The pneumatic tire having the tread pattern shown in FIG. 6 does exhibit an improvement in performances as compared to the pneumatic tire having the tread pattern of FIG. 5, but the improvement is still insufficient.
In view of the aforementioned, an object of the present invention is to provide a pneumatic tire having a block pattern which can provide, in particular, good off-road performance, on-snow performance, and on-road traveling performance, altogether.
A first aspect of the present invention is a pneumatic tire comprising: a circumferential groove extending along a circumferential direction of the tire, and provided within a region which is 10 to 40% of a ground contact width from one ground contact end of a tread in the widthwise direction toward a tire equatorial plane; another circumferential groove extending along the circumferential direction of the tire, and provided within a region which is 10 to 40% of the ground contact width from another ground contact end of the tread in the widthwise direction toward the tire equatorial plane; at least two rows of blocks which, when a region between the pair of circumferential grooves is defined as a center tread region, are divided, in at least the center tread region, by a plurality of transverse grooves extending substantially in a tire axial direction; first reinforcing portions provided integrally with tire-widthwise-direction other side end portions of first blocks located at tire-widthwise-direction one side among the at least two rows of blocks, such that the first reinforcing portions are connected to block side surfaces of the first block and adjacent transverse groove bottom portions; and second reinforcing portions provided integrally with tire-widthwise-direction one side end portions of second blocks located at the tire-widthwise direction other side which are adjacent to the first blocks, such that the second reinforcing portions are connected to block side surfaces of the second block and adjacent transverse groove bottom portions, wherein at least a portion of each first reinforcing portion faces the second block with the transverse groove therebetween in the circumferential direction of the tire and overlaps the second block in the tire widthwise direction; at least a portion of each second reinforcing portion faces the first block with the transverse groove therebetween in the circumferential direction of the tire and overlaps the first block in the tire widthwise direction; and a height of the first reinforcing portion decreases as the first reinforcing portion is distanced from the first block, and a height of the second reinforcing portion decreases as the second reinforcing portion is distanced from the second block.
In accordance with the above-described first aspect, the following excellent effects are achieved.
(1) Because the pair of circumferential grooves is provided in the tread, performance on snow preventing the side slip, the straight forward traveling stability, the hydroplaning performance on wet road surfaces, and the like can be ensured.
(2) A plurality of blocks are formed by the transverse grooves (which may be parallel to or inclined with respect to the tire axial direction) at least at the center tread region. Thus, the basic performances required of a tire for four wheel drive (traveling on rough roads, traveling on ice and snow, and the like) can be ensured.
(3) Because at least two block rows are provided in the center tread region, cornering performance at the time of traveling off-road and at the time of traveling on snow can be ensured.
(4) In a case in which the number of transverse grooves is increased in order to improve the on-snow performances (such as traction or the like), the volume of the block decreases such that the rigidity of the block decreases, and it may be easy for the block to deform. However, in accordance with the present aspect, when a load which could deform the block in the circumferential direction of the tire (in the conventional tire) is applied to the block, the reinforcing portion provided at that block markedly suppresses the deformation of the block. Further, even if the block is on the verge of deformation, that block abuts against and is supported by the reinforcing portion of a block adjacent thereto at the deformation side. Thus, deformation of the block can effectively be suppressed.
Accordingly, while deformation of blocks is effectively suppressed, the number of transverse grooves can be made greater than in the conventional art, and thus, the on-snow performance is markedly improved.
(5) The height of the reinforcing portion decreases as the reinforcing portion is distanced from the block. Thus, when the blocks become worn due to traveling, the edge factor of the blocks gradually increases. Accordingly, a deterioration in the on-snow performances and off-road performances, which deterioration accompanies a decrease in the groove volume of the circumferential grooves and the transverse grooves due to wear, is suppressed. Thus, there is little change in the performances from the time the tire is a new product until the final stages of wear.
(6) Because the reinforcing portions are provided at the end portions of the blocks, as compared with blocks at which no reinforcing portions are provided, the block rigidity at the time the tire is new is high, and the stability and maneuvability on paved roads is improved.
In a second aspect of the present invention, all portions of the first reinforcing portion face the second block with the transverse groove therebetween in the circumferential direction of the tire and overlap the second block in the tire widthwise direction, and all portions of the second reinforcing portion face the first block with the transverse groove therebetween in the circumferential direction of the tire and overlap the first block in the tire widthwise direction.
In accordance with the second aspect, the effect of suppressing deformation of the block is greater than in a case in which (only) a portion of the reinforcing portion faces, in the circumferential direction, the adjacent block.
In a third aspect of the present invention, the height of the first reinforcing portion decreases at a constant rate as the first reinforcing portion is distanced from the first block, and the height of the second reinforcing portion decreases at a constant rate as the second reinforcing portion is distanced from the second block.
In accordance with the third aspect, the height of the reinforcing portion decreases at a constant rate as the reinforcing portion is distanced from the block. Thus, the edge factor can be increased at a constant rate as wear of the block proceeds.
In a fourth aspect of the present invention, the pneumatic tire further comprises center blocks located at the center in the widthwise direction of the tire, and left and right (hand-side) second blocks provided at outer sides of the center blocks in the widthwise direction of the tire, the center blocks, the left second blocks and the right second blocks being provided in three rows at the center tread region, wherein a reinforcing portion, provided at an end portion of each second block at the equatorial plane side in the widthwise direction of the tire, faces the center block with the transverse groove therebetween in the circumferential direction of the tire and overlaps the center block in the tire widthwise direction, and a reinforcing portion, provided at end portions of the center block at the outer sides thereof in the widthwise direction of the tire, faces the second block with the transverse groove therebetween in the circumferential direction of the tire and overlaps the second block in the tire widthwise direction.
In accordance with the fourth aspect, by making the center tread region into three rows of blocks, at least one row of blocks can be disposed on the tire equatorial plane. Thus, the on-road traveling performances (on-road feeling and sense of rigidity) are improved.
Further, at the center tread region, the number of transverse grooves is greater than in a case in which there are two rows of blocks. Thus, traction and braking performance on snow can be improved.
At each of the blocks of the center tread region, when the block receives a load in a direction of deforming the block (along the circumferential direction of the tire), the deformation is markedly suppressed by the operation of the reinforcing portion.
In a fifth aspect of the present invention, the widths of the transverse grooves between the reinforcing portions and the blocks facing the reinforcing portions in the circumferential direction are more narrow than the widths of the other transverse grooves.
In accordance with the fifth aspect, even if the amount of deformation of the block is relatively small, the block can abut the reinforcing portion, and thus, the amount of deformation of the block can reliably be kept small.
For example, when a large force in the tire longitudinal direction is applied at the time of braking or traction or the like, the blocks support one another, which contributes to an improvement in the rigidity of the tread.
In a sixth aspect of the present invention, the groove widths of the transverse grooves between the reinforcing portions and the blocks facing the reinforcing portions in the circumferential direction are set in the range of 0.5 to 4 mm.
In accordance with the sixth aspect, the effect of providing the reinforcing portions can reliably be obtained while a deterioration in performances on wet roads is suppressed.
In a seventh aspect of the present invention, the reinforcing portion has an inclined surface whose angle, with respect to a line normal to the tread surface of the block, is set within a range of 10 to 70xc2x0. It is more preferable that the angle of the inclined surface of the reinforcing portion, with respect to the line normal to the tread surface of the block, is set within a range of 15 to 65xc2x0.
In accordance with the seventh aspect, the effect of providing the reinforcing portions can reliably be obtained while the disadvantage caused by a decrease in groove volume is suppressed.